In recent years, well drilling systems have been demonstrated wherein sensors of various types located close to the drill bit have provided information in real time for control and analysis of the drilling procedure itself and for the evaluation of geological data. Representative information such as hole direction, tool face angle inclination, weight and torque-loading of the bit are clearly important data from which drilling rig operational efficiency depends. Other information, such as electrical resistivity of local strata, natural gamma-ray spectra and vector magnetometer data provide information for assessing the geologic nature of the surrounding strata. These and other parameters can be measured to high precision and there results a telemetry problem in providing a data transmission system which can accommodate an acceptable signal-to-noise ratio at an acceptable data rate. As well as conveying survey data to the surface, this telemetry facilitates transmission of control parameters to the drilling tool itself.
Straightforward electrical transmission across drill string components requires adaptation of those components to provide the required insulated conductors and reliable electrical couplings. These requirements introduce a plurality of vulnerable components in an extremely hostile environment promoting the likelihood of communication failure.
Another transmission means of particular interest exploits the drilling fluid (mud) circulated through the drill string and returned to the surface. The mud pressure can be modulated or an acoustic carrier wave can be developed at the downhole transmitter or at the surface for propagation through the drilling fluid to the acoustic receiver. Modulation techniques of various types have been utilized for impressing information on the carrier for processing at the receiver. Fast mud valves for creating pressure pulses, mud sirens and variants thereof for generating an acoustical carrier are described in a number of works, representative of which are systems discussed in U.S. Pat. No. 4,215,425 and U.S. Pat. No. 4,215,427. Other references to the general state of the art are to be found in Patton et al, J. Petr. Tech., v, Oct. 1977; Gearhart et al, J. Petr. Tech., v., 1980.
It is apparent that the data channel for mud pulse telemetry is exceedingly noisy owing to the mechanical generation of broadband noise and to the drilling fluid circulation system. A signal-to-noise ratio which will sustain acceptable demodulation can be achieved with lengthy pulse integration with data rate reduced accordingly; however, pulse integration times are further limited by the characteristics of the channel. For example, the static pressure level or the baseline of a modulated carrier is subject to drift at a rate which effectively limits pulse width, necessitating a return-to-zero (hereafter "RZ") format for reliable pulse detection. Thus, in any pulse code modulation, variations in a relevant base line parameter (pressure, frequency, phase or the like) is compensable by establishing pulse windows in time separated by no pulse intervals during which the base line parameter is monitored.
In particular, direct pressure modulation requires operation of a mud pressure valve against an ambient pressure of the order of some 10.sup.3 psi to produce a pressure increment of 50-200 psi with a rise and fall time of the order of tenths of seconds for pulse intervals of the order of a few seconds. Reproducibility of valve performance deteriorates with erosion of valve parts and wear on seals consequent to these demands, and power requirements for securing these operating specifications are nontrivial. It is therefore desirable for the pulse code to be efficient in order to reduce the valve duty cycle and total number of valve actuations in order to conserve power and prolong the operating life of the valve. It is also desirable that the pulse code be accurately decoded. U.S. Pat. Nos. 3,519,746 and 4,513,403 disclose correlation techniques that have been developed in the past for this purpose.
In order that conventional parlance be disturbed as little as possible, the use herein of the terms "code", "encode" and "decode" will be understood to refer to the relationship of data symbols to the modulation of the communication channel, e.g., the transformation and its inverse for associating the symbol with its physically realized modulation. Thus "modulation" retains its conventional connotation referenced to a carrier or baseline pressure (carrier of zero frequency). Demodulation is, of course, the inverse of modulation.